There are many applications where a valve is provided in a gas or liquid (collectively referred to as a "fluid") flow path to control the flow of the fluid. One such application is in the ducting of a fume hood where the valve may be in the form of a damper controlling airflow through the hood. An example of such a system is shown in FIG. 1 and is described later. The fume hood system of FIG. 1 is also described in U.S. Pat. No. 4,706,553 entitled "Fume Hood Controller" issued Nov. 17, 1987.
In many prior art systems of this type, airflow through the valve or damper is monitored by an airflow sensor which may by a stand-alone device. A closed loop control is then provided to modulate the valve damper as required in response to outputs from the airflow sensor.
An inherent limitation on such systems is that they have a slow response time. This is because there is some time delay between a change in damper/valve position and the resulting change in airflow being sensed by the sensor. It also takes the sensor some time to sense the new airflow, convey such information to the control and for the control to then make an appropriate adjustment. Such devices are also subject to overshoot and undershoot errors which, as a minimum, increase response time in reaching a desired airflow and may result in a desired airflow not being achievable.
An improvement on such devices in appropriate applications is the use of a Venturi valve. In such valves, airflow is controlled by a cone shaped element positioned in and movable in the valve's orifice to vary the size of an annular-shaped fluid flow path formed in the orifice. Due to the shape of the cone and that of the orifice, the pressure drop across the valve's orifice can be measured by the force exerted on the cone by the difference between the static pressures directly in front of and behind the cone caused by the increased air velocity behind the cone. This effect is similar to the lift force created on a wing by the higher velocity and lower pressure appearing over the top of the wing. The Venturi valve uses this force to act upon a variable rate spring located inside the cone which connects the cone to the valve's shaft. The purpose of this spring is to provide a pressure compensating action such that, for a given position of the valve's shaft, the flow rate of the valve is constant or independent of pressure changes over some range of pressure drops across the valve (such as from 0.6" wc to 3.0" wc).
The result of the Venturi valve's cone and spring pressure compensating action is that there is a specific and fixed relationship or characteristic between the valve's shaft position and the fluid flow through the valve. To the extent that this characteristic is not predetermined for a given valve, it may be empirically determined for the valve in a given application by performing an initial calibration procedure. Once the characteristic is known, all that is required to achieve a desired airflow is to move the valve shaft to the appropriate position for such airflow. Thus, with such a valve, speed of response is limited only by the ability of control electronics to move the shaft to a new position. Accuracy is limited only by the accuracy of the calibration data, the accuracy of the control electronics and the effectiveness of the valve's variable rate spring and cone assembly in compensating for pressure differences. The elimination of the air flow feedback loop also eliminates the potential overshoot and undershoot problem of the prior approach.
Venturi valves of the type described above are currently controlled by comparing the actual flow, determined either by measurement or by operating on the valve shaft position with the valve characteristic, with a signal indicative of desired fluid flow and utilizing the resulting error signal to operate the valve actuator. The problem with such a control is that it is nonlinear, having a different gain for different fluid flows and valve shaft positions. However, since it is desirable for the valve to move quickly when it has large distances to travel so as to achieve rapid response time, while slowing down as the valve approaches the position for the desired flow so as to avoid overshoot, this nonlinearity makes it difficult to achieve the desired control characteristics for all shaft positions. In particular, if the response time is adjusted to avoid overshoot when the valve shaft moves for only a small distance to achieve a desired fluid flow change, the valve responds too slowly in regions where large shaft movements are required to achieve the same fluid flow change; while if the shaft speed is adjusted to achieve reasonable response time in the latter regions, overshoot occurs. Valve controls of this type therefore suffer from instability, less than optimal response time, and inherent overshoot and undershoot problems. Problems of this type may also exist in other types of valves having a control parameter other than shaft position controlling fluid flow.
Further, some valve systems of this type are currently digitally controlled, and may use a microprocessor. While digital control offers enhanced flexibility of design, digital circuits are also more complicated to design, to install and to maintain, particularly for relatively unsophisticated electricians. Further, a digital or microprocessor controlled system is normally slightly slower than a comparable analog system which may result in variation or errors in the length of modulating pulses to, for example, a pneumatic valve controller used for the valve. Such variations are another potential cause of overshoot or undershoot in valve control.
Finally, it is desirable to have an indication of actual fluid flow in some applications of such valves which may be used as a feedback signal for certain system control functions. However, as indicated above, a sensor in the flow path has an inherent time delay. A means for providing such a feedback indication which is not time delayed is therefore required.
It is therefore an object of this invention to provide for the linear control of valve position, and in particular, for the control of a Venturi-type valve, so that optimum control rates may be selected for repositioning of the valve shaft which rates are appropriate regardless of current or final shaft position. More particularly, it is an object of this invention to permit the valve shaft to be moved at a higher rate when the valve shaft has greater distances to travel, thus optimizing valve response time, while permitting the rate of travel of the valve shaft to be slowed when the valve is near its desired end position so as to avoid overshoot or undershoot problems and avoid potential instability. Another object of this invention is to provide a simple analog implementation for achieving the above objects. Finally, it is an object of this invention to provide a non delayed indication of current fluid flow which may be utilized for feedback or other system functions.